In the field of optics and particularly in the field of laser technology it is increasingly frequently desirable to change the direction and/or to vary the intensity of a beam of optical radiation--referred to below briefly as a "light beam"--by means of an external signal. If there are no very demanding requirements as to the speed with which these changes are to proceed this object may be performed by mechanically or electro-mechanically moved optical components, e.g. by pivoting or rotating mirrors or prisms. The object may also be fulfilled by a piezoelectrically controlled change of the width of the air gap between two prism surfaces, at one of which the total reflection of an incident beam is partially suppressed by the closely adjacent other surface. If there are demanding requirements as regards time acousto-optical deflectors and modulators are used. However, if the requirements as regards time are extremely demanding and switching or modulating times of only a few nanoseconds are required, only electro-optical components can be used for this purpose. In this connection Pockels cells may be primarily considered which have almost completely replaced the old Kerr cells which are associated with many disadvantages. In a Pockels cell the refractive indices for the ordinary beam and the extraordinary beam are altered to a differing degree in an electro-optical crystal by the application of an electric field which effects a change in the double refraction of the crystal. In this manner the polarization of a light beam passing through it can be reorientated in dependence upon the length of the crystal and the magnitude of the applied field strength so that a change in direction and/or amplitude of the light beam can occur in an optical crystal element by virtue of its double refraction. Such devices have been used in laser technology in very many different embodiments for different purposes. If they are used within a laser resonator these devices can, for instance, be used in order substantially to change the efficiency of a resonator at a predetermined time within a few nanoseconds or periodically to modulate it in the interval of a resonator period. In the first case so called giant pulses could be produced; in the second ultra-short laser pulses are produced via the so called active phase coupling. In the so called cavity-dumping the entire light energy stored in the resonator can be coupled out at a predetermined time with this device and directed in a predetermined direction outside the resonator. Outside a laser resonator such devices are frequently used for the control of the intensity of the laser beam or for modulation for the purpose of the transmission of communications.
A considerable disadvantage of Pockels cells is the very high voltage (in general many kilovolts) which is necessary for the operation of a Pockels cell in the known apparatus and whose application to the Pockels cell is very difficult to achieve within a few nanoseconds with electronic components.
Other known arrangements which are seldom used (such as, for instance, prisms of electro-optical crystals whose deviation of the light depends upon the magnitude of the applied electric field) have the disadvantage that they are scarcely usable in practice due to the small change in angle of the light beam on the application of even extremely high field strengths.
Logic circuits operating purely optically, such as AND, OR and NOT components are also known which substantially comprise a Fabry-Perot-Interferometer which includes a medium whose refractive index is a non-linear function of the light intensity. However, relatively high light intensities are required for these which must be precisely controlled; furthermore a control by an electrical signal is preferable for many purpose.
U.S. Pat. No. 3,586,416 discloses a light modulator employing an interferometer, in which an incident beam is divided into two partial beams which are recombined at a small angle to produce a spacial pattern of interference fringes at a predetermined position. By providing at or close to that position a gate or mask comprising for instance transparent slots in an opaque support, positioned to allow the fringes to be aligned to the slots, greater than 50% of the output beam will emerge from the system. A Kerr or Pockels cell is provided in one of the partial beam paths of the interferometer. The cell is controlled by an electrical signal to change the optical path length of said one partial beam path, and if the optical path length is changed by a half wave-length, the entire fringe system is shifted half a fringe width. The space-selective gate or mask will then intercept most of the shifted fringe pattern with the result, that very little light now emerges from the system. However, this known light modulator allows modulation only between about 80% and about 20% of the maximum intensity, and the space-selective mask is a delicate component which must be exactly adjusted.